This invention relates to an improved exhaust gas sensor electrical circuit. More particularly, the invention relates to circuitry suitable for use with an exhaust gas oxygen sensor of the type employing first and second metal oxide ceramic elements one of which is more responsive to the partial pressure of oxygen in the exhaust stream from an internal combustion engine than is the other of the metal oxide ceramic elements.
In a known type of exhaust gas oxygen sensor and associated electrical circuit, two metal oxide ceramic elements are electrically connected in series across a DC source of electrical energy. Both of the metal oxide ceramic elements are responsive to variations in their temperature. The temperature response is in the form of a variation in the electrical resistance between spaced lead wires that are embedded in or otherwise connected to the metal oxide ceramic materials. The electrical resistance between the lead wires also varies, to different degrees, as a function of the partial pressure of oxygen in exhaust gases to which the two metal oxide ceramic elements are exposed. These exhaust gases are produced by the combustion of an air/fuel mixture that may, in a feedback fuel control system, be caused to vary from rich to lean and from lean to rich with respect ot the stoichiometric level.
The response to the metal oxide ceramic elements to temperature and to the partial pressure of oxygen in the exhaust gases occurs over a normal temperature operating range that extends from about 350.degree. C. to about 850.degree. C. and which, in some cases, may extend from a lower temperature of 300.degree. C. or less to a temperature as high as about 900.degree. C. The preferred form of the exhaust gas oxygen sensor utilizes an oxygen sensing element that is quite porous and is made from titania, a metal oxide ceramic material responsive to both partial pressure of oxygen and to temperature with a resistance which decreases substantially as its temperature increases. The resistance also decreases substantially as a result of a change from exposure to exhaust gases produced from combustion of a lean air/fuel mixture. A change from rich to lean produces the opposite variation in resistance, that is, a substantial increase.
The second ceramic element in the exhaust gas sensor may also be made from titania material, but preferably is more dense than the oxygen-sensing titania element to lengthen the time rate of response to the second element to variations in the partial pressure of oxygen as compared to the oxygen sensing element. Otherwise stated, the oxygen sensing element should respond quickly to variations in oxygen content of exhaust gases and the second element should respond more slowly, i.e., have a lengthened time rate of response thereto. The response of the second element, usually referred to as a thermistor element, to variations in its temperature preferably is substantially similar to the response of the oxygen-sensing element.
The output signal from the exhaust gas oxygen sensor is a voltage measured between the junction formed between the first and second metal oxide ceramic elements and one of the leads of the DC source of electrical energy. When the air/fuel ratio of the mixture supplied to the internal combustion engine is switched cyclically from rich to lean and from lean to rich typically with a frequency approximately one Hz, then the output voltage signal varies from almost 100% of the source voltage to nearly 0% of this voltage and then back to 100% thereof at a frequency corresponding to that of the variation in the air/fuel ratio. This variation in voltage magnitude is substantially independent of temperature due to the presence of the second metal oxide ceramic element which, as compared to the first element, has little time rate of change of electrical resistance as a function of the variation in partial pressure of oxygen in the exhaust gases.